Various metal oxides, such as stannic oxide SnO.sub.2, indium oxide In.sub.2 O.sub.3, and cadmium stannate Cd.sub.2 SnO.sub.4, have been the most widely used materials for forming transparent, electrically conductive coatings and layers.
The earliest methods of applying these coatings were based on spraying a solution of a metal salt (usually the chloride) on a hot surface, such as glass. In this way, satisfactory transparent, electrically resistive layers were first made for de-icing aircraft windows. However, the spray process produced rather corrosive by-products, hot chlorine and hydrogen chloride gases, which tended to attack the hot glass surface, producing a foggy appearance. U.S. Pat. No. 2,617,745 teaches that this undesirable effect can be mitigated by first applying a coating of pure silica on the glass. However, a silica protective layer is not very effective on glass with a high alkali content and high thermal expansion coefficient, such as common soda-lime glass. In addition, these corrosive by-products attack metal parts of the apparatus, and the metallic impurities, such as iron, may then be deposited in the coating, with deleterious effects on both the electrical conductivity and transparency of the coating.
Another problem has been a lack of uniformity and reproducibility in the properties of the coatings. U.S. Pat. No. 2,651,585 teaches that better uniformity and reproducibility are obtained when the humidity in the apparatus is controlled. The use of a vapor, rather than a liquid spray, as described for example in German Pat. No. 1,521,239, also results in more uniform and reproducible coatings.
Even with these improvements, more recent studies have been made using vacuum deposition techniques, such as evaporation and sputtering, in order to achieve cleaner and more reproducible coatings. Despite the much higher cost of these vacuum processes, the reduction of corrosive by-products and unwanted impurities introduced by the spray methods is felt to be important particularly in applications involving high-purity semiconductors.
The intentional addition of certain impurities is important in these processes, in order to achieve high electrical conductivity and high infrared reflectivity. Thus, tin impurity is incorporated in indium oxide, while antimony is added to tin oxide (stannic oxide) for these purposes. In each case the function of these desirable impurities ("dopants") is to supply "extra" electrons which contribute to the conductivity. The solubility of these impurities is high, and they can be added readily using all of the deposition methods referred to above. Fluorine has an advantage over antimony as a dopant for tin oxide, in that the transparency of the fluorine-doped stannic oxide films is higher than that of antimony-doped ones, particularly in the red end of the visible spectrum. This advantage of fluorine is important in potential applications to solar cells and solar thermal collectors. Despite this advantage of fluorine, most--and perhaps all--commercially available tin oxide coatings use antimony as a dopant. Possibly this is because fluorine doping has only been demonstrated in the less satisfactory spray method, whereas the improved deposition methods (chemical vapor deposition, vacuum evaporation and sputtering) are not believed to have been shown to produce fluorine doping. In addition, a recent report by a committee of experts in the American Institute of Physics Conference Proceedings No. 25, p. 288 (1975), concludes that fluorine equilibrium solubility in tin oxide is inherently lower than that of antimony. Nevertheless, it is noted that the lowest resistivity tin oxide films reported in the prior art are those of U.S. Pat. No. 3,677,814 to Gillery. Using a spray method, he obtained fluorine-doped tin oxide films with resistances as low as 15 ohms per square by utilizing a compound, as a starting material, which has a direct tin-fluorine bond. The lowest resistance in a commercially available tin-oxide coated glass is presently in the range of about 40 ohms per square. When one wishes to obtain coatings of as low as 10 ohms per square, one has heretofore been forced to use the much more expensive materials like indium oxide.